Bead blast activation of carbon nanotube cathode

ABSTRACT

Activation of printed or dispensed carbon nanotube (CNT) film using a particle-blasting technique, also referred to as sandblasting or bead blasting. The process works by sending particles of material at high enough velocity such that when the particles hit the surface, some of the material at the surface is removed. The surface of the printed CNT film is slowly eroded away by the particles from the particle gun. The CNT fibers may be embedded in several layers of the printed layer, so they may not be removed easily.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present invention claims priority to U.S. Provisional ApplicationSer. No. 60/530,584.

TECHNICAL FIELD

The present invention relates in general to field emission devices, andparticularly to field emission from carbon nanotubes.

BACKGROUND INFORMATION

Carbon nanotubes (CNTs) are used as cold cathode electron sources forfield emission applications. These applications include displays (seeU.S. Patent RE38,223; and C. G. Lee et al., “FEDs with CNT on large areaapplications,” Proceedings of the 9^(th) International DisplayWorkshops, p. 1021, Dec. 4-6, 2002, Hiroshima, Japan), x-ray tubes (seeU.S. Patent RE38,223; and G. Z. Yue et al. “Generation of continuous andpulsed diagnostic imaging x-ray radiation using a carbon-nanotube-basedfield-emission cathode,” Applied Physics Letters, Vol. 81, pp. 344-357),microwave devices (Chris Bower et al., “A micromachined vacuum triodeusing a carbon nanotube cold cathode,” IEEE Transactions on ElectronDevices, Vol. 49, No. 8, p. 1478, August 2002), satellite thrusters andneutralizers and other applications requiring a source of electrons. Insome cases, the carbon nanotube film or layer is grown on the substrateusing various chemical vapor deposition (CVD) techniques known in thestate of the art. These films do not generally require an activationprocess and are used as grown. However, there are many advantages tofabricating the carbon nanotube (CNT) cathode by dispensing or printingthe CNT layer. See U.S. Patent Publication No. US-2003-0092207-A1,incorporated by reference herein. These inks or pastes are depositedonto the substrate by screen printing, dispensing, ink-jet printing,spraying, painting or other such means.

These processes have several advantages over growing the CNT materialonto the substrate using CVD techniques. The dispensing and printingprocesses do not require process temperatures much above 450° C.-500°C.; the CNT fabrication process (generally at 600° C. and above) isseparated from the dispensing process. There are many vendors thatsupply many different sizes and characteristics of CNT, so optimalmaterial for field emission applications can be identified. Furthermore,printing and dispensing processes are demonstrated to be low-cost andcan be scaled to large area in high-volume manufacturing environments.These processes are also capable of making CNT films having superiorfield emission properties (low threshold fields, high currentcapability, etc.).

A problem with these printing or dispensing approaches is that theyoften require an activation process. These activation processes includelaser blasting or laser beam activation (see Junko Yotani et al.,“CNT-FED for Character Displays,” Society for Information Display 2004International Symposium Digest of Technical Papers, Vol. 35, Book II, p.828-831, May 26-27, 2004; and also S. Nakata et al., “Fabrication of CNTElectron Source by Simple Stacking for Obtaining Uniform EmissionDistributions,” Society for Information Display 2004 InternationalSymposium Digest of Technical Papers, Vol. 35, Book II, p. 928-931, May26-27, 2004; and W. Rochanachirapar et al., “Effect of laser irradiationon CNT-cathodes in different atmospheres,” Proceedings of the 10^(th)International Display Workshops, p. 1207-1210, Dec. 3-5, 2003, Fukuoka,Japan), exposure to Ar ion plasma (see Yasunori Kanazawa et al.,“Improvement in electron emission from carbon nanotube cathodes after Arplasma treatment,” Journal of Vacuum Science and Technology B, Vol. 22,p. 1342-1344, 3 Jun. 2004), air jet and surface scratching or rubbing(see Kwang-Bok Kim et al., “Efficient electron emissions from printedcarbon nanotubes by surface treatments,” Journal of Vacuum Science andTechnology B, Vol. 22, p. 1331-1334, 3 Jun. 2004), or tape activation(see U.S. Publication No. US-2003-0092207-A1; and Yu-Yang Chang,Jhy-Rong Sheu, and Cheng-Chung Lee, “Method of improving field emissionefficiency for fabricating carbon nanotube field emitters,” U.S. PatentApplication Publication 2002/0104603 A1; and Daniel T. Colbert et al.,“Method for growing continuous carbon fiber and compositions thereof,”U.S. Patent Application Publication 2002/0109086 A1).

Tape activation generally requires an adhesive tape and a roller orlaminator. After the CNT paste is dispensed onto the substrate andprocessed at high temperature (e.g., 300° C.-500° C.) to eliminateorganic binders and solvents and cure the paste, the tape is applied tothe substrate such that the adhesive side of the tape is fixed to thetop of the substrate and the CNT printed pattern. The tape is fixed bylamination or rolling or other means. The tape is then removed from thesubstrate, and during this process the top layer of the CNT paste isremoved, exposing a fresh layer of CNT (not exposed to curingenvironment). The CNT density may also be altered and some of the CNTsmay be aligned vertically to the substrate (parallel to the substratenormal). A problem with tape activation is that:

-   -   1) adhesive material may be left on the substrate,    -   2) it may be difficult to scale to large area and high volume,    -   3) structures on the substrate may be damaged or changed,    -   4) the adhesive layer on the tape may not be uniform over large        area, and    -   5) it may not result in uniform emission properties because of        differences in applied pressure during lamination across the        surface of the substrate, differences in adhesion and structures        already on the substrate that prevent the tape from being        applied to certain areas of the CNT paste.

An example of the last point is shown in FIG. 1. The CNT film is printedonto a conducting line on an insulating substrate. In this case, onlyone pixel is shown, but the same is true for substrates having manypixels. Insulating layers are also printed on the substrate to act asspacer layers on which to attach a grid after the activation process iscompleted. It is difficult to apply the tape near the edges of the gridspacer layers since the tape and lamination rollers are not compliantenough to form around these structures. This results in incompleteactivation of the CNT pixel and non-uniform emission properties of theprinted CNT layer in the pixel area

Laser beam activation may also have the same uniformity issues nearother structures. It may also damage the CNT because of the high heatfrom the laser beam. It may also not be scalable to high volumemanufacturing of large area displays.

Mechanical or surface scratching also has the same problems. Mechanicalbrushes would not be able to reach to the bottom of a well as shown inFIG. 1 to activate a CNT layer printed at the bottom of the well.Mechanical brushes may also adversely change the direction of alignmentof the CNT fibers.

What is needed in the art is an activation process that can be lowercost, can be scaled to high volume and improves the field emissionuniformity in the cathode.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 illustrates tape activation of carbon nanotubes;

FIG. 2 illustrates an embodiment of the present invention;

FIG. 3 illustrates exposure of carbon nanotubes by bead blasting;

FIG. 4 illustrates a comparison of light emitted from a cold cathodeactivated by various methods;

FIG. 5 illustrates an IV graph of cold cathode samples activated byvarious methods;

FIG. 6 illustrates a comparison of light emitted from a cold cathodeactivated by various methods;

FIG. 7 illustrates an IV graph of a cold cathode activated by variousmethods;

FIG. 8 illustrates a method for applying a CNT mixture to a substrate;

FIG. 9 illustrates IV curves;

FIGS. 10A-10C illustrate a process for rastering a bead blaster througha mask to activate a cold cathode; and

FIG. 11 illustrates bead blasting through an insulating layer.

DETAILED DESCRIPTION

Disclosed is a system and method of activation of printed or dispensedCNT film using a particle-blasting technique, also referred to herein assandblasting or bead blasting. The process works by sending particles ofmaterial at high enough velocity such that when the particles hit thesurface, some of the material at the surface is removed. This is similarto the process used to engrave features or text into stone as hard asmarble or granite. Similar processes are also developed to be gentleenough to remove dust particles from the surface of microelectroniccircuits during fabrication of the circuits without damaging thecircuits.

Referring to FIG. 2, particles or beads are shot at the surface from agun using forced air, a centrifugal process or other means. A machineoperable for performing this process is the Alps Engineering Micro BlastMachine BSP-20. This is a table-top machine, but Alps Engineering andother companies make much larger machines for large area and high volumeapplications. Similar machines have been used for barrier rib structureson glass substrates for plasma display panels (see U.S. Pat. No.5,876,542). The particles can be a solid material like glass, silicaSiO₂, alumina Al₂O₃, silicon Si, or other insulator, semiconductor,metallic particles, or powders. The particles can be spherical, cubical,or other shapes, with smooth or rough surfaces. The particles can alsobe wires, whiskers or nanotubes. They can be frozen materials such asfrozen water H₂O or carbon dioxide CO₂ “dry ice”, i.e., materials thatare normally liquid or gas at standard room temperatures and pressures.Dry ice (CO₂) will not leave a residue (it will vaporize to a gas afterhitting the surface). Sand particles (mostly SiO₂) will also work.

The surface of the printed CNT film is slowly eroded away by theparticles from the particle gun. The CNT fibers may be embedded inseveral layers of the printed layer, so they may not be removed easily.As illustrated in FIG. 3, this process then will expose a fresh layer ofCNT and remove some of the top layer of binder material from the printedCNT layer.

To demonstrate advantages of this process, a layer of CNT paste wasprinted using single-wall carbon nanotubes. Double and multi-wallnanotubes can also be used with similar results. The CNT paste wasscreen printed onto the substrate in square pixel patterns. One part ofthe sample was activated by sandblasting as described above to removeonly a top layer of the printed material (top part of the next figure).A small lab sandblaster was used. The center part of the sample was notactivated at all by any method. The other third of the sample wasactivated using the tape method (see FIG. 1). The sample was mounted ina vacuum test chamber and all sections of the sample were tested at thesame time. The sample was mounted in a diode configuration with aphosphor anode as one electrode and the CNT cathode as the otherelectrode. The anode was at ground potential. A pulsed bias voltage wasplaced on the cathode such that electrons from the cathode wereextracted and accelerated to the anode to create light as a result of anelectric field between the cathode and anode electrodes. FIG. 4illustrates that the sandblast method activated the printed CNT cathodematerial to achieve a lower threshold with respect to the tape activatedand non-activated areas of the cathode. At the same potential, morecurrent is extracted from the area activated by sandblasting (leading tomore light coming from the phosphor in this area) compared to the tapeactivated and non-activated areas (almost no light at all).

A hard mask such as a metal foil, or a soft mask such as photoresist,may help define the sandblast pattern and keep the particles fromeroding other areas of the cathode that need to be left intact. Multipleguns can be used or guns may be large to cover large areas of thecathode. The particles can be as large as several millimeters indiameter or as small as several tens of nanometers. Smaller particleswill be gentler and will result in a finer pattern. Larger particleswill be more effective in removing material and will results in fasterprocessing. In the above process, particles of order 0.1 mm (100microns) or smaller diameter were used.

ALTERNATIVE EMBODIMENT NO. 1

As noted previously, CNT material is used that is applied to thesubstrate by screen printing a paste material containing carbonnanotubes. This embodiment shows that the activation process of thepresent invention is excellent for this type of CNT cathode material.

CNT paste may be prepared by mixing the following ingredients. Singlewall CNT material from Iljin Nanotech Co., Korea, may be used in thisformulation.

Preparing the CNT Paste

A recipe for making the paste is given below. A wide range of CNT pastesare possible. CNT: 5-25% Vehicle: 50-90% Glass Frit: 5-25%

Vehicle material acts as the carrier for the powders in the paste and iscomposed of both volatile (solvents) and non-volatile (polymers)organics. An example of a vehicle is model F1016A02 provided by Pierce &Stevens Corp.

Thinner (Terpineol, Dupont 8250) may be added to adjust the viscosity ofthe paste. The mixture may be initially ground and mixed with a mortarand pestle and then transferred to a mechanical stirrer to stir themixture (e.g., 3 hours). The mixture may then be transferred to a threeroll mill to further homogenize the paste. There are many variations onmaking the CNT paste used by those that practice the art. The activationprocess of the present invention works with all of these CNT pasterecipes.

Printing the CNT Paste

After the paste is prepared, the paste may be printed onto a substrate(e.g., glass). Two types of glass substrates may be used.

-   -   1) In one case, the substrate may be coated with a layer of ITO        to provide an electrical conduction path to the printed CNT        pattern. A fine mesh (355 mesh) screen of polyester fiber may be        used to print the CNT paste onto the substrate. The area of the        printed layer may be 3-cm×3-cm The viscosity of the paste may be        adjusted to 50,000-150,000 centipoises (cps).    -   2) In another case, the glass substrate may be coated with a        printed, patterned layer of conducting silver (Ag) paste and a        dielectric glass frit layer, typically used in CNT-based field        emission displays. The printed Ag and dielectric glass frit        layer may be already cured at high temperature by the time the        CNT paste later is printed onto the substrate. The CNT paste may        be printed using a metal stencil mask to pattern the CNT layer        onto the substrate. The viscosity of the paste may be adjusted        to 50,000-150,000 cps.

In both cases, a Presco screen printer made by AMI may be used to printthe CNT paste onto the substrate. Other screen printers will also work.Typical squeegee printing speeds and pressures may be used and are wellknow to those skill in the art of screen printing.

After printing the CNT layer, the substrate may be dried and cured usingstandard processes known to those skilled in the art of screen printingglass frits and Ag patterns. Nitrogen gas may be flowed through the ovenduring the high temperature curing process. Many variations can be usedto dry and cure the CNT paste.

Activation of Printed CNT Cathode

Several samples were prepared as described above. Each was mounted intoan Alps Engineering MBP-20 Micro Blast Machine. Glass beads of 30 microndiameter were loaded into the MBP-20 machine. The machine was programmedto raster the bead spray nozzle back and forth with the same pattern foreach sample. The parameters that were varied were:

-   -   The rate of beads being fed into the gun as noted in revolutions        per minute (rpm) of the feed motor delivering bead stock to the        gun, and    -   The air pressure used to force the beads to the substrate        surface. Several pressures were tried starting at 20 psi (pounds        per square inch). It was discovered that this pressure setting        removed material quickly. The CNT paste material is very soft        after the firing step. Good results were achieved when the gun        was operated at low pressures (about 4.2 psi).        Field Emission Results Of Activation Process

Table 1 shows the results of several samples printed with the 355 meshprocess. Sample p3-a was a control sample (for comparison) that was notactivated by sandblasting but by a standard tape activation process (seeFIG. 1). The approximate amount of CNT material (as determined by thedarkness of the material) that was removed in the process is noted inthe last column. The field emission performance results are also notedas the extraction field needed to achieve 30 mA of current from the3-cm×3-cm square cathode area. Low extraction fields are one figure ofmerit to judge the quality of the cathode; low extraction fields aredesired with all other parameters being equal. By applying the beadblast activation method using a feed rate of 100 and a gun air pressureof 4.2 psi, an extraction field was achieved at 30 mA of current that issignificantly lower than the control sample that is processed using atape activation press. Feed Field at Amount Pressure 30 mA Sample (rpm)(psi) (V/um) Comments P3-a na na 6.1 control glass, taped P3-b 200 207.8 99% CNT removed P3-c 200 7 7.2 90% CNT removed P3-e 100 7 6.3 80%CNT removed P3-d 100 4.2 4.8 20% CNT removed

The field emission current-voltage (I-V) curves for each of the samples,expressed in current density as a function of extraction field, is shownin FIG. 5. The samples were tested in a diode mode with a 2% dutyfactor.

Similar results were achieved with the stencil printed samples on the Agfeedlines. FIG. 6 shows light emitted from a sample that was treated bythe standard tape activation process on the top half and the bead blastactivation process on the bottom half The CNT cathode substrate wasassembled with an anode screen in a diode mode and operated at a 2% dutyfactor. Both halves were subjected to the same field. The brighter half,which was activated by the bead blast process, has a lower thresholdfield and thus delivers more current at the particular current thisimage was taken.

FIG. 7 shows the I-V curves of a sample activated by the bead blast(Micro Blast) method compared to a similar sample prepared by the tape(traditional) activation process. The results show that the bead blastmethod also leads to a lower threshold field.

ALTERNATIVE EMBODIMENT NO. 2

CNT material is also used that is sprayed or dispensed onto the surface.Often this material has little or no binder (see Chili-Che Kuo et al.,“Spray-coated process for preparing CNT-FED cathode,” SID 2004International Symposium Digest of Technical Papers, Vol. 35, Book II, p.825-827, May 26-27, 2004) to hold the CNT onto the substrate relative topaste samples deposited by screen printing. The activation process ofthe present invention works well on this type of CNT cathode material aswell.

Purified singlewall carbon nanotube (SWNT) powders from CarbonNanotechnologies Incorporated, Houston, USA may be used. These SWNTs maybe approximately 1-2 nm in diameter and 1-15 microns in length. Othersingle wall or multiwall carbon nanotubes can also be used. This processalso works well with CNT material mixed with nanopowders andmicropowders of other materials such as glass, SiO₂, Al₂O₃ or otherinsulators, semiconductors or metals and alloys.

Preparation of SWNT Solution

1) Grinding of CNTs

The CNT materials may be ground into shorter lengths to improvedispersion of the CNTs in the solvents. This may permit better controlof material properties. In some cases, satisfactory results may beachieved without grinding A simple ball mill may be used to grind SWNTbundles. The rate of this machine may be approximately 50-60 revolutionsper minute. 0.05-1.0 gram SWNT bundles as well as 40-100 Al₂O₃ balls(5-10 mm in diameter) may be mixed into 200-1000 ml IPA (isopropylalcohol). The mixture may be ground for 1-7 days in order to dispersethe SWNTs. A surfactant (e.g., Triton®X -100, about 1 drop per 100 mlIPA) or similar material can also be added to the mixture in order toachieve better dispersion of SWNTs.

Other solvents can be used instead of IPA (e.g., acetone). Mixtures ofsolvents can also be used. Water or mixtures of water and organicsolvent may also be used IPA is inexpensive, is not extremely hazardousor toxic, and can be evaporated at relatively low temperatures.

2) Ultrasonic Mixing

Because the SWNTs can easily agglomerate (stick to each other), anultrasonic mixing process using a horn or bath may be applied to theSWNT solution after removing from the ball mill to disperse the SWNTsagain before spraying them onto the substrates. Full power may beapplied to the sonicater for 3-5 minutes, until the SWNT solution startsto warm to about 40° C. Other means of applying ultrasonic energy to thesolution may also be used.

3) Spraying of CNT Mixture onto the Substrate

The SWNT may be sprayed onto ITO-coated glass plates in 2-cm×2-cm squarepatterns. A shadow mask may be used to define the pattern of SWNT on theglass. The SWNT solution may be sprayed on various kinds of substratessuch as metal, ceramic, glass, plastics, organic and semiconductors. Thesubstrates may be coated with conducting, insulating or semiconductingpatterned layers to provide electrical conductivity to some areas andelectrical isolation or selected electrical resistance to other areas.These layers may be deposited using printing methods (thick film) or byevaporation, sputtering or other thin film methods. Photolithographypatterning and/or etching processes may be needed for additionalpatterning of these layers. These are standard techniques used in thedisplay and IC industry and are not relevant to this invention.

In order to achieve further uniform and well dispersed SWNT coating onthe substrates, more IPA may be added into the above solution beforespraying. In order to prevent the solution from flowing under the maskto unwanted areas, the substrates may be heated up to 50° C.-100° C.both on front side and back side during the spray process to quicklyevaporate the IPA solvent. Magnets behind the glass may be used to holda metal shadow mask firmly to the glass. The substrates may be sprayedback and forth or up and down several times until the SWNTs cover theentire surface uniformly. The thickness of the SWNTs may beapproximately 1-5 microns. A diagram of the spray set-up is shown inFIG. 8. Ink jet printing or other printing techniques may also be usedto apply the CNT mixture to the substrate.

Activation of Spray-CNT Cathode

Several samples were prepared as described above. Each was mounted intoan Alps Engineering MBP-20 Micro Blast Machine. Glass beads of 30 microndiameter were loaded into the MBP-20 machine. The machine was programmedto raster the bead spray nozzle back and forth with the same pattern foreach sample. The parameters that were used were:

-   -   The rate of beads being fed into the gun as noted in revolutions        per minute (rpm) of the feed motor delivering bead stock to the        gun, and    -   The air pressure used to force the beads to the substrate        surface. Several pressures were tried starting at 20 psi (pounds        per square inch). It was discovered that this pressure setting        removed material quickly, since the CNT was weakly bound to the        substrate surface (no binder was used). Good results were        achieved when the gun was operated at low pressures (less than 3        psi).

Table 2 shows the results of several samples. Sample c-1 was a controlsample that was not activated by bead blasting but by a standard tapeactivation process (see FIG. 1). It was used as a control forcomparison. The approximate amount of CNT material that was removed inthe process is noted in the last column. The field emission performanceresults are also noted as the extraction field needed to achieve 30 mAof current from the 2-cm×2-cm square cathode area. Low extraction fieldsare one figure of merit to judge the quality of the cathode; lowextraction fields are desired with all other parameters being equal.Feed Air Field at Amount Pressure 30 mA Substrate (rpm) (psi) (V/um)Comments c-1 na Na 2.9 control sample, tape activated c-2 100 1.4 3.280% CNT removed c-3 100 2.8 3.6 99% CNT removed c-4 10 2.8 2.7 20% CNTremoved c-5 30 2.8 2.9 60% CNT removed c-6 20 2.8 2.8 40% CNT removedc-7 10 2.8 3.0 20% CNT removed c-8 5 2.8 3.2 10% CNT removed

The field emission current-voltage (I-V) curves, expressed in currentdensity as a function of extraction field, is shown in FIG. 9. The curvewith the lowest extraction field at is c-4, although other curves showsimilar results. At least 3 samples activated by the bead blastingtechnique disclosed here have results that are as good as or better thanthe benchmark taped sample. The samples were tested in a diode mode witha 2% duty factor.

Variations

The bead blast activation process will work with CNT cathodes made fromsingle, double or multiwall carbon nanotubes. It is also possible topattern a CNT cathode in the same process used to activate the cathodeusing a bead blasting process. For example, one can pattern lines ofconducting feedlines that contain CNT fibers using standard screenprinting techniques. Using a mask and a bead blaster, one can activatespecific regions of cathode as defined by the open areas of the mask.

Step 1. Print conducting feedlines containing CNT fibers on a insulatingsubstrate such as glass. See FIG. 10A.

Step 2. Put mask over the conducting feedlines and bead blast throughthe mask. See FIG. 10B.

Step 3. Remove mask, only the areas where the feedline was exposed theblasting beads (areas corresponding to holes in the mask) will beactivated for good field emission. See FIG. 10C. This process worksequally as well if the feedline also was coated with an insulating layersuch as glass frit. See FIG. 11. The bead blasting would drill throughthe insulating layer. When the bottom of the hole reached the feedlinelayer containing the CNT fibers, the blasting activity would activatethis layer inside the hole that was created.

It is important to also note that the angle of attack of the beads orparticles can be perpendicular to the surface (typical), but it can alsobe at other angles off of perpendicular, ranging from 0-90 degrees.

1. A method for enhancing field emission properties of a cold cathode bybead blasting a surface of the cold cathode material.
 2. (canceled) 3.(canceled)
 4. (canceled)
 5. The method as recited in claim 1, whereinthe beads comprise insulating particles.
 6. The method as recited inclaim 1, wherein the beads comprise semiconductor particles.
 7. Themethod as recited in claim 1, wherein the beads comprise metallicparticles.
 8. The method as recited in claim 1, wherein the beadscomprise frozen materials that are normally liquid or gas at standardroom temperatures and atmospheric pressures.
 9. The method as recited inclaim 1, further comprising positioning a mask over the cold cathode toeffect a patterning of the bead blasting.
 10. A method for enhancingfield emission properties of a cold cathode comprising the steps of:applying a cold cathode material, capable of emitting electrons inresponse to an electric field, onto a substrate; and bead blasting asurface of the cold cathode material.
 11. The method as recited in claim10, wherein the cold cathode material comprises carbon.